the major technical problem i see is that this dimension must be some other neutral place, not within obito. Kakashi was in no way surprised by the place because in learning to kamui himself, he had been there before, if he did all that then he would have been within obito before (no innuendo intended), meaning obito would already have had many opportunities to confront kakashi within his personal dimension. Also, another thing that classifies it as a S/T jutsu working in a neutral dimension, is the fact that minatos FTG worked via that dimension, we know that because obito had not shown any other tech at the time but his kamui and his transparency jutsu and yet he was able to clash with minato while minato travelled in this alternate dimension to a point in real world that allowed the travel to be instant. so if not only kakashi has access to it, but minato does as well, i doubt it is a sealing technique, or at least kamui dumps them into a neutral dimension that can be accessed several ways.